1. Field of the Invention
The invention relates to a foamable mixture containing an organically modified organopolysiloxane, processes for preparing the organopolysiloxane and foams which can be produced from the foamable mixtures.
2. Description of the Related Art
Both pure silicone foams and flexible polyurethane foams produced from organic polyols and diisocyanates or polyisocyanates have been known for a long time. However, both groups of materials have specific advantages and disadvantages. Thus, although silicone foams generally have a good high- and low-temperature stability and excellent flame resistance, they at the same time have a comparatively high density and an only very moderate mechanical property profile. Flexible polyurethane foams, on the other hand, usually have excellent mechanical properties. A disadvantage of many polyurethane foams is, however, burning behavior which is unsatisfactory for many applications and can be compensated, if at all, only by means of large amounts of added flame retardants.
The use of silicone-polyurethane copolymers, i.e. polysiloxanes which also contain polyurethane and/or urea units, makes it possible to develop new types of foams which have novel combinations of properties which can be tailored precisely to the respective application. Thus, foams which, in particular, have good mechanical properties in combination with a significantly improved burning behavior compared to conventional polyurethane foams can be produced in this way.
WO 03/080696 describes silicone foams which can be produced from particular hydroxyalkyl- and/or aminoalkyl-functional polysiloxanes and diisocyanates or polyisocyanates. Crosslinking of the silicones occurs here during foam formation. Water, which reacts with the isocyanates which are used in excess to liberate carbon dioxide and form urea units, serves as blowing agent.
WO 03/080696 describes two processes for producing foams. In one process, the hydroxyalkyl- and/or aminoalkyl-functional siloxane is firstly emulsified in water and the emulsion obtained is subsequently reacted with diisocyanates or polyisocyanates. In the second process, the hydroxyalkyl- and/or aminoalkyl-functional siloxane is firstly reacted with an excess of the diisocyanate or polyisocyanate to form an isocyanate-functional siloxane which is then mixed with water in a second process step and thereby foamed.
DE 41 08 326 C1 describes silicone foams which can be produced by reaction of hydroxyalkyl-functional polysiloxanes with diisocyanates or polyisocyanates. The siloxane foams are produced using methods comparable to those described in WO 03/080696.
In addition, the reaction of hydroxyalkyl- or aminoalkyl-terminated polysiloxanes with diisocyanates or polyisocyanates is known from further literature references, including U.S. Pat. No. 5,512,650 or WO 97/40103. However, this reaction has been described not for producing foams but exclusively for producing elastomers or prepolymers for hot melt adhesive or sealant applications. In addition, the compounds described there are, owing to their high molar masses and the very high viscosities associated therewith, unsuitable for use in a process for producing foams from prepolymers, in which crosslinking of the prepolymers is to occur only during foam formation and at low temperatures.
Disadvantages of all processes according to the prior art are the complex handling of multicomponent systems and the possible health hazards associated with the isocyanate content, which have to be ruled out by specific measures.